Add .pre-commit-config.yaml

Install pre-commit.com and run `pre-commit install` in your local repo
and it'll automatically run vimdoc for you whenever you commit.

See google/vimdoc#123 for more details.

Author: dbarnett

.pre-commit-config.yaml

@@ -0,0 +1,6 @@
+# See https://pre-commit.com for more information
+repos:
+- repo: https://github.com/google/vimdoc
+  rev: v0.7.1
+  hooks:
+  - id: vimdoc